The present invention relates to a motor control technique for suppressing a fluctuation of an output of a motor, and in particular, to a technique for controlling a motor having a characteristic in which a magnitude of the output fluctuation changes in accordance with a level of the output of the motor. Moreover, the present invention relates to a technique for suppressing vibration generated in a motor by suppressing an output fluctuation of the motor.
For tightening of fine screws used for assembling products such as precision apparatus, a motor-driven type screw tightening apparatus is used which is capable of accurately controlling an output torque thereof.
However, a motor has a cogging torque, which impedes accurate torque control. Namely, when there is unevenness in a lamination state (i.e., permeability) of the core of the motor, the cogging torque is generated in accordance with a rotational angle of the motor even in a state in which a voltage or an electric current applied to the motor is zero. Due to the effect of the cogging torque, an output torque fluctuates at a frequency corresponding to a rotational speed of the motor×the number of poles in the motor.
Moreover, there is a factor impeding the torque control of the motor except for the cogging torque described above. For example, when there is unevenness in a coil winding of the motor, a torque fluctuation is brought about in accordance with a rotation of the motor. This torque fluctuation increases substantially in proportion to a level of a motor applied current. Then, since a torque fluctuation component due to the unevenness in the coil winding is superimposed on a torque fluctuation component due to the effect of the cogging torque, the fluctuation in the output torque is increasingly made greater. In addition thereto, there is a torque fluctuation due to dimension errors or fabrication errors of parts constituting the motor.
The unevenness in the core's permeability and the coil winding generates a sinusoidal waveform torque fluctuation basically during one rotation of the motor. However, a torque fluctuation is generated so as to draw a complex curve due to variations of individual motors in some cases. Moreover, when a voltage or an electric current applied to the motor (i.e., the output torques) is changed, a manner of generating the torque fluctuation in accordance with the rotational angle of the motor is changed in some cases.
Moreover, there is a problem that vibration is generated in the motor due to a cyclic increase and decrease of the cogging torque in driving the motor.
Therefore, in some cases, a method is employed in which correction for suppressing an output fluctuation and vibration of the motor is performed onto a command value which is input, and energization of the motor corresponding to the corrected command value is performed (refer to Japanese Patent Laid-Open No. 2005-288589 and Japanese Patent Laid-Open No. 2001-37287).
Japanese Patent Laid-Open No. 2005-288589 discloses a motor having a memory in which correction data for suppressing an output fluctuation are stored. In accordance with this technique, the correction data as well are automatically switched according to a replacement of the motor.
Further, Japanese Patent Laid-Open No. 2001-37287 discloses a motor control apparatus having a memory in which correction data for a rotational speed command to suppress the vibration due to the cogging torque or the like are stored.
However, in the screw tightening apparatus disclosed in Japanese Patent Laid-Open No. 2005-288589, one correction data group determined on the basis of a torque fluctuation coefficient when a voltage or an electric current of a predetermined value is applied to the motor is merely stored in the memory. Namely, as described above, in Japanese Patent Laid-Open No. 2005-288589, it is unconsidered that, when the output torque of the motor is changed, a magnitude and a generating manner of the torque fluctuation in accordance with the rotational angle of the motor.
Further, in Japanese Patent Laid-Open No. 2001-37287, in a case where a rotational speed command of the motor is corrected in order to suppress vibration of the motor, plural correction data groups in accordance with plural rotational speeds are stored in the memory. Then, Japanese Patent Laid-Open No. 2001-37287 discloses that a correction value for an input rotational speed command is found by linear interpolation from the plural correction data groups. However, in the same way as in Japanese Patent Laid-Open No. 2005-288589, it is unconsidered that a magnitude and a generating manner of the torque fluctuation are changed in accordance with the output torque of the motor.